Aortic tears with or without dissection may occur following aorto-coronary artery bypass grafting (CABG). Herein we report 4 patients who developed large false aneurysms of ascending aorta following CABG. The sequence of events leading to formation of the aortic false aneurysm in patients #1-3 appears to be: 1) initiation of a limited aortic dissection beginning at an aorto-coronary bypass anastomosis; 2) rupture of the outer partition of the false channel; and 3) progressive enlargement of the false aneurysm, the wall of which was adventitial fibrous tissue with or without adjacent adherent parietal pericardium. In patients #1 and #3, the aneurysm involved the right side of the ascending aorta and compressed the right atrium and superior vena cava; in patient #2, the aneurysm involved the left side of the aorta and compressed the left atrium and pulmonary trunk. In none of these 3 patients was an aneurysm of the ascending aorta suspected clinically. Patient #1 developed substernal chest pain 3 days before death (37 days after CABG) and a pericardial friction rub was recorded. This man died with features of the low cardiac output syndrome. Patients #2 and #3 died from progressive worsening of chronic congestive heart failure and in each, greater than or equal to 1 aorto-coronary conduit was totally occluded. In patient #3, Dressler's syndrome developed early after CABG and during the next 40 months she received prednisone almost daily. Patient #4 had pseudoxanthoma elasticum. After CABG, she developed mediastinitis and died from consequences of a generalized infection. At necropsy, the metallic rings which had been placed to mark the aortic anastomosis of each of the 2 conduits had eroded through the wall of aorta to be visible from its interior. The proximal portions of the conduits were surrounded by thrombus and both conduits were occluded. The rupture of the wall of aorta in this patient appears to have been the result of both mediastinitis and the underlying weakened aorta from pseudoxanthoma elasticum.